Cell Division

• Chromosome structure

– Made of chromatin (mix of DNA andprotein)

– Only visible during cell division

Cell Division• Chromosome structure

– The DNA in a cell is packed into an elaborate,multilevel system of coiling and folding.

Double helix Nucleosome Helical fiber Chromosome

Cell Division

• Chromosome structure

• Before a cell divides, it duplicates all of itschromosomes, resulting in two copies called sisterchromatids

• When the cell divides, the sister chromatids separatefrom each other

Cell Division

• The Cell Cycle

– Eukaryotic cells that divide undergo an orderlyseries of events called the cell cycle.

Consists of two distinct phases: Consists of two distinct phases:

Interphase Interphase (I) (I) - cell grows & copies- cell grows & copies itsitschromosomes in preparation for cellchromosomes in preparation for celldivisiondivision

Mitotic phase (M)Mitotic phase (M) - cell division occurs - cell division occurs

Cell Division

• Mitosis– Division of a nucleus into 2 daughter nuclei– Consists of four distinct phases:

• Prophase• Metaphase• Anaphase• Telophase

Cell Division

• Prophase

– chromosomes condense & formvisible chromatids

– centromere starts to form(region of sister chromatid &microtubule attachment)

– nuclear membrane breaks down

Cell Division

• Metaphase– chromosomes align on the

metaphase plate along thecenter of the cell

– nuclear membrane gone

– microtubules attach to an areaof the centromere called thekinetochore

Cell Division

• Anaphase

– individual chromatidsseparate to oppositeends of the cell

Cell Division

• Telophase– chromosomes reassemble at each

“pole”

• nuclear membrane reforms• cytoplasm divides

(cytokinesis)

– chromosomes uncoil, becomeextended & again cannot beidentified

Cell Division

• Comparison of animal and plant cell division

– Cytokinesis• Animals

– Furrowing (contracting ring) of cell membrane

• Plants– Cell plate formation– Cell membrane formation– Cell wall formation

Cell Division• Regulation of cell division

– Normal plant and animal cells have a cell cycle control system

– Mechanisms of cell division regulation include:• contact inhibition• anchorage dependence• growth factors

Cell Division

• Out of control cells

– Cancer is caused by a breakdown in control of the cell cycle

– Cancer cells• cells become deregulated and immortal (transformation)• loss of contact inhibition and anchorage dependence• grow in unorganized lumps called tumors

Cell Division• Cancer tumors

– Tumors that are surrounded by a basement membraneare called benign.

• can often be removed by surgery

– Tumors that invade surrounding tissues are calledmalignant.

• surgical removal often incomplete

– Metastasis - spread of transformed cells to locationsdistant from the original site

Cell Division• Types of cancer treatments

– Radiation and chemotherapy disrupt cell division.

– Target rapidly dividing cancer cells as well as normal cells.

• those of scalp (causing hair loss)• intestinal lining (nausea / loss of appetite)• bone marrow (causing suppression of immune system)

Cell Division• NEW Types of cancer treatments

– Boosting immune system as a whole.

– Targeting the immune system against tumor-associatedantigens.

– Using antibodies to target anti-cancer drugs to attack cancercells more exclusively.

Cell Division

• The Genetics of Cancer

– Proto-oncogenes

• Normal genes that can become oncogenes(“cancer causing genes”)

• Found in many animals• Code for growth factors that stimulate cell division

– For a proto-oncogene to become an oncogene, a mutationmust occur in the cell’s DNA

Cell Division

• The Genetics of Cancer

– Tumor suppressor genes

• Normal genes that control DNA repair

• Mutation of these genes often result in failure ofDNA repair which may result in cancer.

Cell Division• Cancer has complex causes and risk factors

– Increasing age• perhaps due to accumulated mutations or exposure to carcinogens

– Cancers associated with viruses.• viruses may cause cancer by inserting oncogenes into host DNA• Human T-cell Leukemia Virus (HTLV)• Human Papilloma Virus (associated with cervical cancer)

– Physical and chemical carginogens.

– Dietary factors (high-fat, low-fiber diet = “bad”)

Cell DivisionMeiosis

• Definition– Reduction division– Gamete formation by means of 2 cell divisions resulting in

haploid cells

• Significance– Variation– Sexual reproduction allows for new genetic combinations.

Cell DivisionMeiosis

• Homologous chromosomes– Chromosomes come in matched pairs– Their number is characteristic of

species(human - 46; chimpanzee - 48; fruit fly - 8)

• Somatic cells (typical body cells)– Humans have 46 chromosomes– Two different sex chromosomes, X and Y– 22 pairs of matching chromosomes, called

autosomes

Pair of homologous chromosomes

Sister chromatids

Cell DivisionLife cycle of a sexual organism

• Sequence of stages leading fromthe adults of one generation to theadults of the next

• Humans are diploid organisms– cells contain two sets of

chromosomes– gametes are haploid, having

only one set of chromosomes

Haploid gametes (n = 23)

Meiosis

Diploidzygote

(2n = 46)

Multicellulardiploid adults

(2n = 46)

Mitosis and development

Egg cell

Sperm cell

Fertilization

Cell Division

Meiosis (My what-is?)

• Meiosis produces gametes for sexual reproduction

• Two consecutive divisions occur, meiosis I & meiosis II,preceded by interphase.

• Crossing over occurs (leads to variation)

Cell DivisionMeiosis (My what-is?) Meiosis I: Homologous

chromosomes separate

Microtubules attachedto Chromosomes

Sister chromatidsremain attached

Cleavagefurrow

Sisterchromatids

Tetrad Centromere

Anaphase I Telophase Iand Cytokinesis

Sites of crossing over

Spindle

Prophase I Metaphase I

Homologouschromosomespair and exchangesegments

Two haploid cellsform: chromosomesare still double

Tetrads line up Pairs of homologouschromosomessplit up

Cell DivisionMeiosis II:

Sister chromatids separate

Anaphase II Telophase IIand Cytokinesis

Sister chromatidsseparate

Haploid daughter cellsforming

Prophase II

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Metaphase II

Cell Division

Genetic Variation

• Offspring of sexual reproduction are genetically different fromtheir parents & from one another

– Independent assortment of chromosomes

– Random fertilization

– Crossing over

Cell Division

Genetic Variation

• Independent assortment of chromosomes

• Every chromosome pairorients independentlyof the others duringmeiosis

Metaphase ofmeiosis II

Gametes

Metaphase ofmeiosis I

Cell Division

Genetic Variation

• Random fertilization

– Egg cell is fertilized randomly by one sperm,leading to genetic variety in the zygote.

Cell Division

Genetic Variation

• Crossing over

– Homologous chromosomesexchange genetic information

– Genetic recombination occurs

Prophase Iof meiosis

Metaphase I

Metaphase II

Gametes

Cell DivisionComparing Mitosis and

Mitosis

Prophase Iof meiosis

Metaphase I

Metaphase II

Gametes